Yet Another Use For B-Type Natriuretic Peptide?

Abstract & Commentary

By Andrew M. Luks, MD, Pulmonary and Critical Care Medicine, University of Washington, Seattle, is Associate Editor for Critical Care Alert.

Dr. Luks reports no financial relationship to this field of study.

Synopsis:This single-center trial demonstrated that the percentage change in plasma B-type natriuretic peptide levels during the course of a two-hour spontaneous breathing trial improved the predictive value of the trial on extubation outcomes.

Source: Chien JY, et al. Crit Care Med. 2008.36(5): 1421-1426.

B-type natriuretic peptide (BNP) is a neurohormone secreted by the heart whose levels are elevated in acute heart failure and also in left- and right-ventricular dysfunction. In the acute setting, BNP measurements have been shown to assist in determining whether patients' respiratory complaints are cardiac or non-cardiac in origin. Because cardiac dysfunction can limit attempts to separate patients from mechanical ventilation, Chien and colleagues conducted this single-center trial to determine whether plasma BNP levels drawn before and after a two-hour spontaneous breathing trial (SBT) improved the predictive value of the SBT for extubation success.

With the exception of patients with a tracheostomy, all patients being considered for separation from mechanical ventilation for the first time were eligible to participate and were included in the study if they demonstrated improvement in the underlying cause of respiratory failure, had a PaO2 > 60 mm Hg on FIO2 < 0.40 and PEEP < 5 cm H2O as well as the absence of fever, altered mental status and hemodynamic instability. Included patients were placed on a two-hour T-piece trial. Patients who met standardized criteria were deemed to have passed their SBT and were extubated. Extubated patients were then followed for 48 hours and were reintubated if they met one of several pre-specified criteria, such as worsening hypoxemia (SaO2 < 90%, PaO2 < 60) or PaCO2 > 50 mm Hg. Plasma BNP levels were drawn at the beginning and end of the 2-hour SBT. ROC curves were used to determine the percentage change in BNP which had the best combination of sensitivity, specificity, positive and negative predictive value and diagnostic accuracy in predicting extubation success. This information was then tested in a separate validation cohort using a protocol similar to that described above.

In the testing cohort, 41 of 52 patients passed their SBT and were extubated. Eight patients (20%) required reintubation for a variety of reasons. There were no statistically significant differences in pre-SBT plasma BNP levels between the SBT failure, extubation failure and extubation success groups. The median percent change in plasma BNP levels during the course of the SBT was significantly lower in the extubation-success group (0.69%, range -8.8%-10.7%) than in the SBT-failure group (20.1%, range 5.9%-73.6%) and the extubation-failure group (32.7%, range 25.7%-50.8%). A change of plasma BNP level < 20% from baseline had the best combination of sensitivity, specificity, positive and negative predictive value and diagnostic accuracy in predicting extubation success (91%, 88%, 97%, 70% and 91%, respectively). Combining the percent change in plasma BNP with the SBT result improved the positive predictive value of extubation success from 80 to 97%. Similar improvements in the positive predictive value were found when the 20% change in BNP cut-off was tested in the validation cohort.

Commentary

Plasma BNP measurements may eventually lead to improvements in our ability to predict extubation success, but before clinicians start ordering the test on all patients whom they are thinking of separating from the ventilator, some caution is warranted. Aside from recognizing that this was a small, single-center study, it is important to recall exactly what this trial demonstrated. The pre-test BNP alone is not sufficient for diagnostic purposes as there were no differences in the baseline measurements between the failure and success groups; in fact, plasma BNP levels overlapped considerably between the SBT-failure, extubation-failure and extubation-success groups. Instead, the BNP must be measured before and after the SBT, which, most importantly, was 2 hours in duration. It is only under these circumstances that BNP was found to increase predictive capability for extubation success.

The two hour duration of the SBT is particularly important, because this study's results differ from those of another study1 that found that the change in plasma BNP during a one-hour SBT could not differentiate between extubation failure and extubation success. Clearly, not all patients that we look to separate from the ventilator require two-hour SBTs before we can declare them fit for ventilator separation. Rather than considering the use of plasma BNP in conjunction with the SBT on all patients, attempts should be made to define clinical situations in which these measurements might be useful, such as patients who have been intubated for more than a specified number of days or who have failed previous attempts at extubation.

Reference

  1. Mekontso-Dessap A, et al. B-type natriuretic peptide and weaning from mechanical ventilation. Intensive Care Med. 2006;32:1529-1536.